Well, I guess I am in a category that makes the 5DIII a sort-of welcome camera for me. I'm retired, and my shooting has been only to produce prints that I occasionally exhibit and sell. I sprung for a camera and lenses that I could not really justify a number of years ago, and the camera was great for my purposes: I bought a 1DMkII and filled the bag with Canon's 100-400 IS, 16-35 2.8, 500 f/4 IS, 15mm fisheye, 100mm 2.8 macro, and 28-135 IS (replaced with the Tamron 28-300 VR). Oh, yeah, almost forgot the Canon 50mm f/2.
While I do mostly city and street shooting, I also use the 500mm for the occasional bird shot. I also do panos and in high contrast settings I do HDR, using brackets of 3 or 5 shots. Many of my shots are in buses and subways, Grand Central Terminal, churches, etc., so I valued the high (for that time, at least) ISO of 1600 and 3200. With very careful use of 3rd party NR programs I've been able to denoise and keep sufficient detail for attractive prints in those cases.
The 1DII is on its 3rd shutter box after 2 very premature failures, and I would eventually have sprung for a 1DV or so when the 1DII finally gave up the ghost. However, with Canon's announcement of the 1DX at roughly 1/3 more than the 1D series would cost, I knew that I could not justify the cost of a 1DX. The Nikon D800 had me drooling, except for my substantial investment in Canon glass. The 36mp also would mean that despite the drop of my 1.3 crop factor, I would be able to get the same bird pix and have enough pixels to crop and bring the bird to the same size in the resultant frame.
Now, with the announcement of the 5DIII, I have a reasonable replacement for the old 1DII when it dies. While I wish it were a 36mp camera, the 5DIII's 22mp full frame will still allow some cropping. From viewing samples at high ISO, I see that I could use the 5DIII at much higher ISO than I can use the 1DII for my frequent low light shooting.
I am very pleased to see bracketing of up to +/- 3 EV, and using 2, 3, 5 or 7 frames, which makes the 5DIII very useable for my HDR shooting. I am sure, also, that the AF and exposure capabilities of the 5DIII are far superior to my long-in-the-tooth 1DII, and I will be glad to use 14 bit depth color.
Not pleasing is the $3500 price, a full $1000 more than the 5DII's. Nor is the $495 price I saw on the vertical grip, almost a necessity for me as I have big hands, big lenses and like the added support the grip gives me, not to mention the convenience of the extra controls for vertical shooting.
All in all, though, I'm relieved to have the 5DIII arrive for when the 1DII finally croaks.